A photopolymerizable composition fundamentally contains a photopolymerization initiator and an addition polymerizable compound having at least two ethylenically unsaturated bonds in the molecule (hereinafter, the polymerizable compound is referred to as "polyfunctional monomer"). When the composition is irradiated by light, the composition is hardened to change the tackiness thereof, and becomes insoluble in the solvent. By utilizing these properties, these photopolymerizable compositions have been widely used for photography, printing, surface processing of metals, inks, etc. The utilization methods and application examples of such a composition are described, for example, in J. Kosar, Light Sensitive Systems, pages 158-193, published by J. Wiley & Sons, New York (1965) and K.I. Jacobson and R.E. Jacobson, Imaging Systems, pages 181-222, published by J. Wiley & Sons, New York (1976).
Also, recently, as an image forming method utilizing a photopolymerizable composition, an image forming system utilizing light-sensitive microcapsules containing a photopolymerizable composition in the microcapsules has been proposed. For example, JP-A-57-124343, JP-A-57-179836 and JP-A-57-197538 (the term "JP-A" as used herein indicates an "unexamined published Japanese patent application" disclose a method of forming dye images by imagewise exposing the coloring sheet having a coated layer of microcapsules containing a dye and a photopolymerizable composition composed of a vinyl compound and a photopolymerization initiator, superposing the coloring sheet onto an image-receiving sheet, and applying pressure to the whole assembly.
Also, an attempt of spectrally sensitizing the photopolymerizable composition up to the visible light wavelength region and forming digital images using a laser as a light source and an application of such a photopolymerizable composition to full color photographic light-sensitive materials has been investigated.
For example, in Nihon Shasin Gakkai-shi (Journal of The Society of Photographic Science and Technology of Japan), Vol. 49, No. 3, page 230(1986) and Kino Zairyo (Functional Materials), pages 48-60, (September, 1983), describe a spectral sensitization method of photopolymerizable compositions for laser recording. Also, JP-A-59-189340 describes a method of spectrally sensitizing an organic peroxide initiator with an organic dye. Furthermore, EP-A 223587 discloses organic boron anion salts of organic cationic dyes.
In particular, a method of using the organic boron compound anionic salt of the organic cationic dye as a photosensitive initiator, has a wide selection range for cationic dye compounds and is useful in designing a photopolymerization initiator composition sensitive to optional wavelengths.
However, by such a method, the photopolymerization initiator composition obtained, having sensitivity to visible light through the light sensitivity thereof, is still unsatisfactory. Thus, in EP-A-223587, the sensitivity is increased by combining the aforesaid composition with an N,N-dialkylaniline derivative, in particular 2,6-diisopropyl-N,N-dimethylaniline. However, the light sensitivity obtained by using the aforesaid combination is yet insufficient.
For photopolymerizabion initiators having light sensitivity to mainly ultraviolet light, various methods for improving the sensitivity have hitherto been known.
For example, a method is known of combining an aminophenyl ketone and an active methyl compound or an amino compound (as described in U.S. Pat. No. 3,661,588), a combination of Michler's ketone and benzophenone (as described in U.S. Pat. No. 3,682,641), a combination of a 2,4,5-triphenylimidazolyl dimer and an organic mercapto compound (as described in U.S. Pat. No. 3,479,185), a combination of an aromatic ketone or a polynuclear quinone and a mercaptoimidazoline or a mercaptobenzoxazole (as described in JP-A-53-702), and a combination of a p-dialkylaminostilbene derivative, a hexaarylbiimidazole, and a mercaptobenzoxazole or a mercaptobenzimidazole (as described in JP A-59-56403).
However, a high sensitivity imparting technique for a novel combination of an organic cationic dye and an organic boron compound anionic salt photopolymerization initiator having light sensitivity to light of longer wavelength than visible light has not yet been known.